The Choice of Wave-lengths for Achromatism in Telescopes

Abstract

DURING the past summer, in the Transactions of the Optical Society, Mr. J. W. Gifford has directed attention anew to the question as to the two wave-lengths most suitable for achromatising telescope objectives. Mr. Gifford has constructed telescopes in which the paraxial focal length for the D and E lines is the same, and, judging by the performance of such objectives, he considers such a choice superior to the more customary choice of the C and F lines. By a different path I have been led to a similar judgment, at least for telescopes of large aperture.